William E. Fowler

Results of vacuum cleaning techniques on the performance of LiF field-threshold ion sources on extraction applied-B ion diodes at 1-10 TW

Uncontrolled plasma formation on electrode surfaces limits performance in a wide variety of pulsed power devices such as electron and ion diodes, transmission lines, radio frequency (RF) cavities, and microwave devices. Surface and bulk contaminants on the electrodes in vacuum dominate the composition of these plasmas, formed through processes such as stimulated and thermal desorption followed by ionization. We are applying RF discharge cleaning, anode heating, cathode cooling, and substrate surface coatings to the control of the effects of these plasmas in the particular case of applied-B ion diodes on the SABRE (1 TW) and PBFA-X (30 TW) accelerators. Evidence shows that our LiF ion source provides a 200-700 A/cm/sup 2/ lithium beam for 10-20 ns which is then replaced by a contaminant beam of protons and carbon. Other ion sources show similar behavior. Our electrode surface and substrate cleaning techniques reduce beam contamination, anode and cathode plasma formation, delay impedance collapse, and increase lithium energy, power, and production efficiency. Theoretical and simulation models of electron-stimulated and thermal-contaminant desorption leading to anode plasma formation show agreement with many features from experiment. Decrease of the diode electron loss by changing the shape and magnitude of the insulating magnetic field profiles increases the lithium output and changes the diode response to cleaning. We also show that the LiF films are permeable, allowing substrate contaminants to affect diode behavior. Substrate coatings of Ta and Au underneath the LiF film allow some measure of control of substrate contaminants, and provide direct evidence for thermal desorption. We have increased lithium current density by a factor of four and lithium energy by a factor of five through a combination of in situ surface and substrate cleaning, substrate coatings, and field profile modifications.

Optical and pressure diagnostics of 4-MV water switches in the Z-20 test Facility

We are studying the behavior of self-breaking, high-voltage water switches for the Z refurbishment project. In Z-20, three or four water switches in parallel are charged to 4 MV in /spl sim/220 ns. The water gap between switch electrodes is 13-15 cm, and the enhancement of the positive and negative electrodes is varied to study time-evolution of the breakdown arcs, current sharing, and switch simultaneity. In addition to the standard electrical diagnostics (V,I), we are looking at one or more of the switches during the breakdown phase with two optical diagnostics: a streak camera and a fast framing camera. The streak camera has /spl sim/1-ns resolution, and the framing camera provides seven frames with >5 ns exposure times. For identical electric fields, the streamers originating on the positive electrode form earlier and move more rapidly than the streamers originating on the negative electrode. We observe four distinct phases in the closure of the water switches that depend on the macroscopic electric fields in the water: 1) No streamers propagate at E-fields below /spl sim/100 kV/cm from positive electrodes or voltages below /spl sim/140 kV/cm for negative electrodes; 2) streamers propagate with constant velocity between 100 and /spl sim/300 kV/cm; 3) above 300 kV/cm, the streamer velocities become linearly proportional to the electric field; 4) above 600 kV/cm, the velocity of streamers from the negative electrodes appears to saturate at /spl sim/100 cm//spl mu/s. The velocity of the streamers from the positive electrode continues to increase with E-field, reaching /spl sim/1% of the speed of light when the switch reaches closure.

Operation and Performance of the First High Current LTD at Sandia National Laboratories

A High Current Liner Transformer Driver (LTD) laboratory is operating at Sandia National Laboratories with an LTD/100 cavity running in rep-rate mode. With over one thousand shots completed the output of the LTD/100 is in excellent agreement with circuit model, SCREAMER [1] simulations, and exceeding the original operating predictions. Operating at a rep-rate of 1/30 Hz and a charge voltage of 85 kV, the LTD/100 is achieving a mean peak current of 514.6 kA plusmn 4.8 kA with a rise time of 60.6 ns plusmn 1.7 ns. With improvements to the support systems of the LTD/100 cavity it will be feasible to achieve a rep-rate of 1/15 Hz or less. The high current, short pulse capabilities combined with good reliability shows that LTD technology is a great candidate for use in future pulsed power applications.

High Current Fast 100-NS LTD Driver Development in Sandia Laboratory

During the last few years Sandia is actively pursuing the development of new accelerators based on the novel technology of linear transformer driver (LTD). This effort is done in close collaboration with the High Current Electronic Institute (HCEI) in Tomsk, Russia, where the LTD idea was first conceived and developed. LTD based drivers are currently considered for many applications including future very high current Z-pinch drivers like ZX and IFE (Inertial Fusion Energy), medium current drivers with adjustable pulse length for ICE (Isentropic Compression Experiments), and finally relatively lower current accelerators for radiography and x-pinch. Currently we have in operation the following devices: One 500-kA, 100-kV LTD cavity, a 1-MV voltage adder composed of seven smaller LTD cavities for radiography, and one 1-MA, 100-kV cavity. The first two are in Sandia while the latter one is still in Tomsk. In addition a number of stackable 1-MA cavities are under construction to be utilized as building blocks for a 1-MA, 1-MV voltage adder module. This module will serve as a prototype for longer, higher voltage modules, a number of which, connected in parallel, could become the driver of an IFE fusion reactor or a high current Z-pinch driver (ZX). The IFE requirements are more demanding since the driver must operate in rep-rated mode with a frequency of 0.1 Hz. In this paper we mainly concentrate on the higher current LTDs: We briefly outline the principles of operation and architecture and present a first cut design of an IFE, LTD z-pinch driver.

High Current Linear Transformer Driver (LTD) Experiments

Sandia Laboratories are actively pursuing the development of new accelerators based on the novice technology of Linear Transformer Driver (LTD) [1,2,3]. LTD based drivers are considered for many applications including future very high current Z-pinch ICF (Inertial Confinement Fusion) drivers like ZX and Z-pinch IFE (Inertial Fusion Energy). The high current LTD driver experimental research is concentrated on two aspects; first to study the repetition rate capabilities, life time and jitter of the LTD cavities, and second to study how a number of cavities behave and add their energy, power and voltage output in a voltage adder configuration assembly.

A 1-MV, 1-MA, 0.1-Hz linear transformer driver utilizing an internal water transmission line

Sandia National Laboratories is investigating linear transformer driver (LTD) architecture as a potential replacement of conventional Marx generator based pulsed power systems. Such systems have traditionally been utilized as the primary driver for z-pinch wire-array experiments, radiography, inertial fusion energy (IFE) concepts, and dynamic materials experiments (i.e. ICE).

Linear Transformer Driver (LTD) development at Sandia national laboratory

Most of the modern high-current high-voltage pulsed power generators require several stages of pulse conditioning (pulse forming) to convert the multi-microsecond pulses of the Marx generator output to the 40–300 ns pulse required by a number of applications including x-ray radiography, pulsed high current linear accelerators, Z-pinch, Isentropic Compression (ICE), and Inertial Fusion Energy (IFE) drivers. This makes the devices large, cumbersome to operate, and expensive. Sandia, in collaboration with a number of other institutions, is developing a new paradigm in pulsed power technology; the Linear Transformer Driver (LTD) technology. This technological approach can provide very compact devices that can deliver very fast high current and high voltage pulses. The output pulse rise time and width can be easily tailored to the specific application needs. Trains of a large number of high current pulses can be produced with variable inter-pulse separation from nanoseconds to milliseconds. Most importantly, these devices can be rep-rated to frequencies only limited by the capacitor specifications (usually is 10Hz). Their footprint as compared with current day pulsed power accelerators is considerably smaller since LTD do not require large oil and de-ionized water tanks. This makes them ideally fit for applications that require portability. In the present paper we present Sandia Laboratorys broad spectrum of developmental effort to design construct and extensively validate the LTD pulsed power technology.

Recyclable transmission line (RTL) and linear transformer driver (LTD) development for Z-pinch inertial fusion energy (Z-IFE) and high yield.

Z-Pinch Inertial Fusion Energy (Z-IFE) complements and extends the single-shot z-pinch fusion program on Z to a repetitive, high-yield, power plant scenario that can be used for the production of electricity, transmutation of nuclear waste, and hydrogen production, all with no CO{sub 2} production and no long-lived radioactive nuclear waste. The Z-IFE concept uses a Linear Transformer Driver (LTD) accelerator, and a Recyclable Transmission Line (RTL) to connect the LTD driver to a high-yield fusion target inside a thick-liquid-wall power plant chamber. Results of RTL and LTD research are reported here, that include: (1) The key physics issues for RTLs involve the power flow at the high linear current densities that occur near the target (up to 5 MA/cm). These issues include surface heating, melting, ablation, plasma formation, electron flow, magnetic insulation, conductivity changes, magnetic field diffusion changes, possible ion flow, and RTL mass motion. These issues are studied theoretically, computationally (with the ALEGRA and LSP codes), and will work at 5 MA/cm or higher, with anode-cathode gaps as small as 2 mm. (2) An RTL misalignment sensitivity study has been performed using a 3D circuit model. Results show very small load current variations for significant RTL misalignments. (3) The key structural issues for RTLs involve optimizing the RTL strength (varying shape, ribs, etc.) while minimizing the RTL mass. Optimization studies show RTL mass reductions by factors of three or more. (4) Fabrication and pressure testing of Z-PoP (Proof-of-Principle) size RTLs are successfully reported here. (5) Modeling of the effect of initial RTL imperfections on the buckling pressure has been performed. Results show that the curved RTL offers a much greater buckling pressure as well as less sensitivity to imperfections than three other RTL designs. (6) Repetitive operation of a 0.5 MA, 100 kV, 100 ns, LTD cavity with gas purging between shots and automated operation is demonstrated at the SNL Z-IFE LTD laboratory with rep-rates up to 10.3 seconds between shots (this is essentially at the goal of 10 seconds for Z-IFE). (7) A single LTD switch at Tomsk was fired repetitively every 12 seconds for 36,000 shots with no failures. (8) Five 1.0 MA, 100 kV, 100 ns, LTD cavities have been combined into a voltage adder configuration with a test load to successfully study the system operation. (9) The combination of multiple LTD coaxial lines into a tri-plate transmission line is examined. The 3D Quicksilver code is used to study the electron flow losses produced near the magnetic nulls that occur where coax LTD lines are added together. (10) Circuit model codes are used to model the complete power flow circuit with an inductive isolator cavity. (11) LTD architectures are presented for drivers for Z-IFE and high yield. A 60 MA LTD driver and a 90 MA LTD driver are proposed. Present results from all of these power flow studies validate the whole LTD/RTL concept for single-shot ICF high yield, and for repetitive-shot IFE.

Cleaning techniques for applied-b ion diodes

Measurements and theoretical considerations indicate that the lithium-fluoride (LiF) lithium ion source operates by electron-assisted field-desorption, and provides a pure lithium beam for 10-20 ns. Evidence on both the SABRE (1 TW) and PBFA-II (20 TW) accelerators indicates that the lithium beam is replaced by a beam of protons, and carbon resulting from electron thermal desorption of hydrocarbon surface and bulk contamination with subsequent avalanche ionization. Appearance of contaminant ions in the beam is accompanied by rapid impedance collapse, possibly resulting from loss of magnetic insulation in the rapidly expanding and ionizing neutral layer. Electrode surface and source substrate cleaning techniques are being developed on the SABRE accelerator to reduce beam contamination, plasma formation, and impedance collapse. We have increased lithium current density a factor of 3 and lithium energy a factor of 5 through a combination of in-situ surface and substrate cleaning, impermeable substrate coatings, and field profile modifications.

Temporally shaped current pulses on a two-cavity linear transformer driver system

An important application for low impedance pulsed power drivers is creating high pressures for shock compression of solids. These experiments are useful for studying material properties under kilobar to megabar pressures. The Z driver at Sandia National Laboratories has been used for such studies on a variety of materials, including heavy water, diamond, and tantalum, to name a few. In such experiments, it is important to prevent shock formation in the material samples. Shocks can form as the sound speed increases with loading; at some depth in the sample a pressure significantly higher than the surface pressure can result. The optimum pressure pulse shape to prevent such shocks depends on the test material and the sample thickness, and is generally not a simple sinusoidal-shaped current as a function of time. A system that can create a variety of pulse shapes would be desirable for testing various materials and sample thicknesses. A large number of relatively fast pulses, combined, could create the widest variety of pulse shapes. Linear transformer driver systems, whose cavities consist of many parallel capacitor-switch circuits, could have considerable agility in pulse shape.